The invention relates to the field of semiconductor processing. More particularly the present invention relates to etch back methods for forming sensor contacts during thin film semiconductor processing.
The microelectromechanical systems (MEMS) are being manufactured using process steps often found in traditional semiconductor processes. MEMS fabrication services are becoming widely used in many desirable developmental variations. The use of MEMS technology often presents difficult challenges when integrating MEMS devices into and with compatible semiconductor devices and processes. The semiconductor processes cover many types of devices and materials. One such semiconductor device and process is complementary metal oxide silicon (CMOS) technology. The CMOS process has been traditionally used for fabricating fast low power digital devices. Most MEMS devices are analog type devices. Complete system designs often require the speed and accuracy of modern digital computer processing systems that are coupled to the real world using analog input and output devices. Complete system designs lead to the integration of digital devices and analog devices on a chip with the advantage of an economy of scale. However, such integration of different devices and the corresponding different process steps must be accomplished with inherent compatibility. Many analog devices operate using gold connector contacts because gold is a good electrical conductor that is also non-corrosive and durable. Aluminum is a good conductor, but is highly corrosive, and not desirable for use as an exposed conducting contact. Gold is a large atom, and gold atomic migration through the lattice structures of semiconductor devices often leads to a decrease in the mean time between failure as gold atoms function as an impurity when migrating from an original deposition site. Though highly conductive, gold and silver impurities near the gate junctions of metal oxide silicon (MOS) transistors can lead to premature failures. Hence, in CMOS semiconductor circuits, often polysilicon, aluminum, and tungsten are used as conductors to avoid the migration problem when using gold or silver.
A sensor contact metal, such as gold or tungsten, can be deposited in a contact via well leading to a semiconductor device in a preexisting semiconductor circuit. For example, a sensor contact metal can be deposited over the contact via well with a potentially corrosive analog sensor then being deposited onto the sensor contact metal. The sensor contact metal can then be covered by a deposited protection layer to protect the sensor and contact metal from corrosion when the sensor is exposed to the environment. As a preexisting example, a silicon substrate may have an aluminum conducting etch run that is covered by an insulation layer such as silicon dioxide. During photoresist application, mask exposure and development, a contact via is formed through the photoresist. Photoresist is usually applied by spinning a coating onto a silicon wafer. The silicon dioxide layer is then etched in the location of the photoresist via to form the contact via through the silicon dioxide layer. The photoresist layer is then removed leaving the silicon dioxide layer over the aluminum conductor excepting for the contact via through the silicon dioxide layer. The formation of the contact via through the silicon dioxide layer to the conductor etch run of the semiconductor is an initial starting process point for depositing sensor contact metal upon a buried conductor etch run, prior to then depositing the sensor on the contact metal. The metal sensor contact is deposited as a layer and then patterned. The metal sensor contact should have profile that mates to the profile of the contact via well and extends up and over the insulation layer for contact with the sensor. Often, the metal contact will have a dimple over the contact via well as the metal is deposited evenly over the contour of the contact via well. Various processes have been used to accurately form the profile of the metal sensor contact during well filing.
The tape liftoff process applies an adhesive tape to the deposited sensor contact metal layer. The adhesive tape makes adhesive contact with the sensor contact metal except over the contact via where the dimple is created in the surface of the metal sensor contact layer. As the contact metal being is deposited down into the contact via well, a surface dimple is created. As the adhesive tape is pulled away from the metal sensor contact layer, the contact layer is removed, except where the dimples are located. Hence, the metal sensor contact survives and remains in the contact via wells. The tape liftoff process is imprecise in forming a metal sensor contact profile and creates ragged edges and stresses in the metal contact, leading to separation failures. Liftoff patterning processes require stepped slopes in the contact wells and constrain the metalization layer to small thicknesses. The Liftoff processes are incompatible with good step coverage and deposition techniques, such as sputtering.
The subtractive process also first deposits a metal sensor contact layer. Patterned photoresist portions are formed over the contact wells, exposing the metal contact layer but not over the contact well. The metal sensor contact layer is removed by a dissolving solution. The metal sensor contact layer is dissolved save the protected metal sensor contacts under the patterned photoresist portions. Then, the pattern photoresist portions are removed exposing the metal sensor contacts that have upwardly extending flanges created on the side walls of the metal contact via and lying upon the insulating layer. The problem with subtractive process is that during the metal sensor contact layer removal step, the metal sensor contacts are undercut under the edges of the pattern photoresist portions leading to imprecise metal sensor contact profiles and flange formation. The metal sensor contacts may also fail to sufficiently adhere to the subsequently deposited sensor.
The chlorobenzene liftoff process uses a single photoresist layer to create large sized sensor contact profiles, the flanges of which can be large. The chlorobenzene liftoff process creates a lip in the photoresist layer that can be damaged during sputtering or heated depositions leading to imprecise formation of the sensor contact profiles. Chemical hazards are disadvantageously created when using exotic and unfamiliar chemicals, such as chlorobenzene, to modify the photoresist.
The multiple layer photoresist process uses multiple layers of photoresist that when respectively repeatedly applied, exposed and then developed, create a thick photoresist via through which the metal sensor contact is deposited to create a unique gold contact profile. The multiple layer photoresist process suffers from the repeated photoresist steps and requires very accurate process controls.
As such, conventional techniques for contact formation disadvantageously suffer from imprecise formations leading undesirable profiles of the metal sensor contact. Often, the metalization layer, including the metal contact can have undesirable contours, such as the metal contact dimples. Conventional etch back methods have been used to remove undesirable surface contours of previously patterned layers. The etch back methods are used for ensuring continuous step coverage and for reflattening the surface for further high resolution photolithography. That is, the etch back method is applied to previously patterned layers. In the case of the CMOS planar etch back method, a metal contact layer, such as tungsten, is deposited over the contact well creating a dimple in the metal layer over the contact well. Because further processes may require substantially flat surfaces, the dimple is removed by a planar etch back process. An insulating layer, such as glass, is reflowed by heat, onto a metal layer. Phosphosilicate glass is applied by chemical vapor deposition and can be reflowed at high temperatures of about 700C. The reflowed glass layer is then etched back to expose the metal layer having surviving portions of the reflowed glass in the dimples. Next, the metal layer is etched back down to the insulation layer where the contact well is then filled with the metal and the surface is then substantially flat. The tungsten layer is effectively patterned into the wells solely by the preexisting lithography. Next, the metal layer is again deposited on a flat surface forming a metalization layer with a flat surface. The flat metalization surface can then be etched to pattern the metal layer without having the contact well dimples. This CMOS planar etch back process provides a metal contact profile that has no dimples. However, the CMOS planar etch back process disadvantageously required two metal deposition processes and two metal etching processes, and results in flat metal sensor flanges that may be unsuitable for connection to MEMS sensors. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a method for forming a metal contact in a contact well.
Another object of the invention is to provide a method for forming in a contact well, a metal contact having upwardly extending metal contact flanges.
Yet another object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple.
Still another object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple, and an upwardly extending metal contact flange.
Still a further object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple, and an upwardly extending metal contact flange for connecting to a sensor.
A further object of the invention is to provide a method for forming in a contact well, a metal contact having a metal contact dimple, and an upwardly extending metal contact flange using polymethylmethacrylate (PMMA) etch back.
Yet a further object of the invention is to provide a method for forming in a contact well, a gold contact dimple, and an upwardly extending metal contact flange using PMMA etch back.
The method is directed to the fabrication of metalized well contacts, such as gold well contacts, for electrical connection between semiconductor microcircuits and microelectromechanical systems (MEMS) devices and sensors, using standard metalization and etch processes with a minimum of subsequent photolithographic processing tools and steps. The method can be performed on variously sized substrates. The method can be used in a variety of fabrication processes for integrating MEMS devices and sensors with semiconductor devices, and is particularly well suited for integrating chemical sensors with conventional metal oxide silicon (MOS) semiconductor processes, such as complementary metal oxide silicon (CMOS) processes. The method can be applied to MEMS devices integrated with conventional semiconductor processes, such as CMOS processes, that can not tolerate a heavy metal, such as a gold metal that acts as an impurity and leads to failure of many silicon devices.
In the preferred form, a complementary metal oxide silicon (CMOS) semiconductor process device, such as a CMOS amplifier having etch run connection under a metal contact well in an insulating layer, is connected to an organic sensor applied to a metal sensor contact in the metal contact well. In the preferred form, the contact metal is gold for electrochemical stability in the presence of a chemical sensor. The use of gold offers good electrical conductivity and high non-corrosiveness. The semiconductor device can be made on large diameter wafers and hence the method offers the potential of an economy of scale when integrating semiconductor processes with inherently incompatible MEMS devices and chemical sensors.
The method is particularly adapted to forming a metal contact with a desirable profile for secure contact with a corrosive chemical sensor. Particularly, a patterning layer, such as a photoresist (PR) layer, is deposited for forming a larger sized patterned contact via over the insulating layer via, for creating upwardly extending metal contact flanges. The patterned contact via effectively increases the well depth and width, while the sidewalls of the patterned contact via provide a bottom surface from which the flanges upwardly extend. The metalization contact layer, such as a gold layer, is deposited over the patterned contact via and insulation contact via in the insulation layer then forming the metal contact with the upwardly extending flanges and with a dimple in the metal contact layer over the contact via using a single metalization deposition step. After metalization, a thick planarization layer, such as a thick layer of PMMA is deposited for filling in the dimple. The planarization layer is then etched back exposing the metalization layer while the dimple and the upwardly extending flanges remain covered with the PPMA. The metalization layer is then removed save the metal contact protected by the PMMA within the contact dimple. The photoresist layer and the remaining PMMA in the contact dimple are then removed to expose the metal contact including the upwardly extending flanges. The flanges extend upward about the height of the PR layer. Hence, the PR layer is used to form the profile of the upwardly extending flanges of the metal contact. With the metal flanges extending upwardly, a chemical sensor or MEMS device can be deposited onto or connected to the upwardly extending flanges of the metal contact. The upwardly extending flange portion of the metal contact and the overall size and shape of the metal contact profile can be precisely formed. The metal contact is suitable for electrical contact between chemical sensors and the underlying semiconductor devices using a minimum number of process steps compatible with existing semiconductor processes. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.